This invention relates to the field of optical to electrical signal transducer devices and their fabrication in the form of an indium material inclusive field effect transistor.
High data rate optical communications systems require high speed optical receivers for information transmittal. These receivers must include an integral high speed optical detector to convert the optical signal to an electrical signal. The semiconductor indium gallium arsenide (InGaAs) when grown lattice matched or near lattice matched to indium phosphide (InP) is a desirable material for such receivers due to its internal electron speed and its efficient absorption of radiation in the 1.3-1.6 micrometer wavelengths favorable to optical fiber communications. The presence of metal electrodes on the front surface of these devices, however, reduces their optical collection efficiency through shadowing. Backside illumination of such detectors is therefore preferred. Electrical contact must then be made to the front surface of the semiconductor device, and illumination applied to the back side.
Semiconductor PIN photodiodes are currently the most widely used detector for this high data rate and other forms of optical communications. Inherently, however, these diodes must comprise semiconductor material of both n-type (electron rich) and p-type (hole rich) regions. Although this material requirement is compatible with bipolar transistor technology, it is highly inconsistent with the now predominant Field Effect Transistor (FET) technology. In order to be integratable with such FEE circuits, a detector must be fabricated as a unipolar device. Photoconductors are unipolar detectors but suffer from slow response and often have high dark current electrical characteristics--which reduces the available signal-to-noise ratio. Metal Semiconductor Metal (MSM) detectors are back-to-back Shottky diodes typically formed as interdigited metal fingers (gates). These devices are fully compatible with FET processing but have no internal gain.
There is therefore a widely perceived need in the electronic art for a practical optical-to-electrical transducer or photodetector or optical receiver device that can be directly integrated into currently used integrated circuit devices, a photodetector that is fully compatible with the fabrication processes and physical and electrical properties of currently used integrated circuit families. The photo field effect transistor inventions of the present patent document and the above-identified and incorporated by reference herein patent documents are believed to provide desirable answers for this need.
The U.S. Patent art indicates the presence of considerable inventive activity, especially with respect to the MESFET (metal semiconductor field effect transistor) device and its use as an electrical circuit element. Prior patent activity appears however not to have extended into the area of photo optical transducing use of the MESFET device as is disclosed herein.
Previous MESFET related patents include the patent of U.K. Mishra et al, U.S. Pat. No. 5,180,681, which is concerned with the fabrication of a high current high voltage breakdown field effect transistor of the MESFET type, including the fabrication of such devices with gallium arsenide material. In addition the invention of S. Inmamura et al, is U.S. Pat. No. 5,204,278, discloses the use of Group III-V periodic table semiconductor compounds such as gallium arsenide in the MESFET device configuration including a Shottky gate electrode arrangement. In addition, the invention of E. Kolesar, Jr., U.S. Pat. No. 4,989,063 is of general background interest with respect to the present invention in the sense that it discloses the use of an epoxy adhesive during a semiconductor device fabrication sequence.
Neither the application of the MESFET configuration to optical transducing devices nor the improved configuration of a MESFET device for this usage is known to have been accomplished in the U.S. Patent prior art, however.
Several publications from the technical literature are also of interest as background information with respect to the present invention. These publications include an early discussion of the compound materials FET device as a photodetector in the article "Light-Induced Effects in GaAs F.E.T.S." by J. Graffeull et al which appears in the publication "Electronics Letters" Vol. 15 No. 14, Jul. 5, 1979. Although this article is concerned with GaAs FET devices it does not involve the substrate removal and backside illumination concepts of the present invention.
The article "Potential of CCDs for UV and X-ray plasma diagnostics" by J. R. Jamesick et al which appears in the publication "Review of Scientific Instruments 56(5), May 1985 is also of this background interest since it discusses the use of backside illumination techniques in the silicon-based integrated circuit art. Since the Jamesick discussion in focused on CCD and silicon technology and not on the signal gain generating, compound materials and substrate removed aspects of the present invention it appears also to be of limited background interest with respect to the present invention.
The article "Picosecond HEMT Photodetector" by T. Uneda et al which appears in the Japanese Journal of Applied Science, Vol 25 No. 10, October 1986, discusses a gallium arsenide based high speed photodetector of the FET type however this discussion also does not extend to the areas of substrate removal and backside illumination. The article "Effect of Signal-Modulated Optical Radiation on the Characteristics of a Modfet by H. Mitra et al which appears in the publication Applied Physics A56, 335-341 (1993) discusses the response of a Gallium Arsenide compound material FET device to a modulated optical signal and provides an analytical model for such response. The Mitra article is however concerned with frontside received illumination.